Process of making alkali subsilicates



Reissued Feb. 4, 1941 UNITED STATES PATENT OFFICE PROCESS OF MAKINGALKALI SUBSILICATES Clarence. W. Burkhart,

Lansdowne, Pa., and Walter 8. Bills, liaddou Heights, N. 1., assignorsto Pennsylvania Salt Manufacturing Company, Philadelphia, 2a., acorporation of Pennsylvania 19 Claims.

Our invention relates to a novel process of manufacturing soluble alkalisilicates, and more particularly it relates to an economic process forthe production of alkalisilicates, of an alkali content not less thanthe silica content, by the direct reaction of caustic alkali and silicawithout dissolution of the caustic alkali or the fusion thereof.

One object of the present invention is to provide a method by which thevarious soluble alkali silicates may be easily and economicallymanwfactured without resort to the expensive equipment and laborioussteps required in either the wet digestion or the fusion processpreviously employed in the production of these compounds.

A further object of the invention is to furnish a process of makingalkali subsillcates by the reaction of solid caustic alkali withpowdered silica, in contradistinctlon to the previous processes wherethe reaction is carried out either in solution or by fusion.

Still another object is to provide a process by which granular orpulverulent soluble alkali silicates may be obtained in a free-flowingmarketable form without the necessity of expending energy and time ingrinding or comminutlng the product to useful size.

A still further object is to provide a process for the manufacture ofhigh grade, free-flowing, stable alkali silicates of low hydration andof any desired ratio of alkali to silica from one to one upwards; forexample, the metasilicate, the sesquisilicate, the orthosilicate, ormixtures thereof, as contrasted with the variety of silicate productsnow available on the market that contain large and varying percentagesof water of crystallization.

Other objects will be apparent from a consideration of the specificationand claims.

The processes heretofore employed in the production of alkali silicates,whose alkali content is not less than the silica content, known, andreferred to herein, as alkali subsilicates, have been time-consuming,laborious, and expensive, and have required the use of expensiveequipment. The subsilicates have previously been manufactured by fusionmethods or by wet digestion methods or a combination of the two.

In the fusion method, an alkali carbonate and silica are heated togetherto high temperatures substantially above the fusion point of the system,and the fused product after cooling is dissolved in water and correctedto the proper alkali-silica ratio. It is then necessary to concentratethe solution. remove the silicates therefrom by crystallization,comminute, and dry the crystals. In

such a process, the plant and equipment are costly, the repair chargesare high due to slagging effect on the furnace linings, and the fuelconsumed is expensive.

In the wet digestion method, silica or waterglass is rendered soluble bydigestion with a concentrated solution of caustic alkali, and thesolution is evaporated and cooled to a suitable crystallizationtemperature. It is then seeded, agitated, and cooled to remove therefromthe heat generated by the crystallization. The crystals are removed,comminuted, and dried. If water-glass is employed, it is obtained byfusing silica with an alkaline carbonate, or an alkali sulphate andcarbon, followed by a special extraction process to render the silicatesoluble. It is obvious that in this process, the evaporation costs arehigh, the layout of the plant is extensive, and the necessary digesterequipment is expensive.

The process of the present invention departs markedly from the previousprocesses since neither fusion nor wet digestion is involved. Inaccordance with the process of the invention, solid caustic alkali andfinely divided silica are reacted directly by mixing at a temperaturebelow the melting point of the caustic alkali. The reaction may becarried out in a simple apparatus, such as a suitable externally-heatedmixing device, to insure agitation of the mass during the reaction. Theproduct from the mixer is available for use in the trade and containswater corresponding substantially to the monohydrate. A product ofimproved appearance and of very low hydration may be obtained by asocalled dressing" process in which the product from the mixer is heatedat a somewhat elevated temperature but below the melting point of r thesilicate. This product is designated herein as a technically anhydrousmaterial, although it may contain a fraction of one molecule of water ofcrystallization. The process in the mixer can be completed in less thanfifteen minutes and the total time required in producing the technicallyanhydrous product may be less than one hour.

It will be seen that the process of the present invention eliminates thedifllculties and expense of both the wet digestion and fusion methodspreviously employed in the manufacture of alkali subsilicates. Thepresent process avoids the multiple adjustments and readjustments ofcomposition, hydration, and concentration, as well as the seeding,crystallization, separation of the crystals, and drying required in thewet digestion method. By the use of ordinary non-specialized equipment,the process also obviates the need of the special fusion furnaces ofcomplicated construction, of the high temperature rotating reactionvessels provided with special mechanical agitators, and of the speciallinings required by the fusion process.

The process of the present invention is applicable for the production ofalkali silicates corresponding to any desired ratio of alkali to silicawhere the alkali oxide content is equal to or greater than the silicondioxide content. For example, a silicate corresponding to themetasilicate, the sesquisilicate, the orthosilicate, or mixturesthereof, may be formed by mixing the desired ratio of ingredients.

Ii. the ratio of NaaO to $102 is 2 to 1, a silicate is formed, after thereaction in the mixer, which is from 99% to 100% soluble. The ratio ofthe two oxides in the compound corresponds to the orthosilicate and asubstantially pure technically anhydrous orthosllicate (Na4Si04) isobtained when the product of the mixer is subjected to the dressingstep. When the ratio of NB-aO to S10: is 1.5 to l, a compound is formedin the mixer in which the ratio of NMO to $101 corresponds to thesesquisilicate. After dressing, a technically anhydrous sesquisilicate(NaoSizOr) is obtained. If a compound in which the ratio 01 alkali oxideto silicon dioxide approaches the metasilicate is desired, 1.25 parts ofNazO are mixed with 1 part of S10: and reacted in the mixer asdescribed. This product may then be further heated to produce atechnically anhydrous product. It will also be clear that if otherratios of alkali oxide to silicon dioxide are desired, the amounts ofmaterials added to the mixer may be adJusted accordingly. Thus mixturesof the various silicates may be obtained. It will be obvious that othercaustic alkalis, for example caustic potash, may be substituted inequivalent amounts for the caustic soda specifled.

The caustic alkali (sodium hydroxide or potassium hydroxide) may beadded to the mixer either in a ground or flake condition. The silica maybe of any desired purity and of any suitable fineness to promote thereaction, and may be either an anhydrous or a hydrated silicon dioxide.The natural amorphous iorms such as opal and diatomaceous earth or thenatural crystalline forms, such as rock crystal, sand, flint, sandstone,or any other variety of quartz can be used. By-produet amorphous orcrystalline forms of silica, hydrated or otherwise, are also suitablesources of silica. Since in general a high purity product is desired, arelatively pure silica will be used, but if the purity of the finishedsilicate is not a requisite, impure silica may be used, provided thatthe impurities do not react at the temperature and under the conditionsof the reaction to form insoluble silicates.

As previously stated. the reaction takes place between thesolid causticsoda and the powdered silica. The reaction proceeds satisfactorily bydry mixing the ingredients. In order to promote the reaction, lubricatethe mixture, and protect the apparatus, a small amount of water may beadded either to the caustic alkali or the silica. In no case, the amountof water added is sufficient to dissolve the alkali used. If water isadded, it will in general not exceed 7% of the total weight of thesilica and caustic alkali.

The mixing device in which the reaction between the caustic alkali andsilica is brought about is preferably externally heated to a point wherethe reaction is sufficiently exothermic to cause completion thereof in arelatively short time. The temperature of the reactiondepends on thenature of the silica material. For example, with properly activatedsilica, the reaction will take place at room temperature. A temperatureof the reacting ingredients of about 175 C. or higher will bring aboutthe reaction with naturally occurring silica in a very short timealthough lower temperatures may be employed. When a. temperature of 175C. is employed in the reacting mass, the temperature rises to 200 C.-210C., due to the heat liberated by the reaction. Temperatures higher thanthis may be employed if desired, as long as the point of fusion ofcaustic alkali is not reached. In general, the temperature of themixture of materials employed will be between 175 C. and 250 C.

As previously pointed out, the mixture of the caustic alkali and thesilica particles is preferably agitated during the heating. The mass,due to the reaction, is converted into a viscous flowable material,which in a very short time becomes a plastic mass. When the temperatureis reached where the reaction becomes noticeably exothermic, the wateradded, if any, is liberated as steam. The plastic mass soon solidifies,and due to the mechanical action of the mixer, a granular or pulverulentfree-flowing product is obtained. The product is suitable for use by thetrade and corresponds substantially to a silicate monohydrate,containing an amount of free caustic alkali, depending on the ratio ofmaterials used.

If desired, in order to improve the appearance and quality of theproduct, the granular material cf the mixer may be dressed by furtherheating it as a. somewhat higher temperature, care being taken not toapproach the melting point of the product. Usually. a temperaturebetween 300 C. and 600 C. will be employed and excellent results areobtained when a temperature in the neighborhood of 450 C. is used. Arotary drier heated indirectly by any suitable source of fuel isapplicable for this treatment and a carbon-dioxide-free atmosphere maybe maintained therein if an especially pure product is desired. Atechnically anhydrous free-flowing, white granular or powdered productis obtained after the heat-treatment in, for example, thirty to sixtyminutes.

In a typical case, 60 parts of very finely divided silica, 100 parts ofpowdered caustic soda, and, if desirable, 10 parts of water are placedin a mixer fitted with a stirring device, and heated. Within a fewminutes, for example six minutes, when the mixer is at an elevatedtemperature when the materials are placed therein, the mixture reaches atemperature of about 175 C. and becomes a viscous flowable mass; in afurther two to two and one-half minutes, a vigorous reaction takesplace, the temperature rises to 200 C. to 225 C. and especially in thecase when water has been added, an evolution of steam takes place and aplastic material is formed. In about a further minute of time, thematerial commences to change from a plastic to a granular condition andthis change is completed in from one and one-half to two minutes. Theproduct is characterized by good scouring, detergent, and abrasiveproperties; can be used by the trade; approaches a metasilicate incomposition; and contains less than unreacted S: and less than unreactedNaOH.

It a technically anhydrous product is desired, the product from themixer is placed in a drier at approximately 450 C. for about forty-fiveminutes. A completely soluble product is obtained which consistssubstantially of equal parts 01' metasilicate and sesquisilicate.

Ii 2. higher purity metasilicate is desired, the dressed product fromthe drier is treated with the necessary amount of water-glass ofsuitable composition to bring the ratio of alkali oxide to silica tothat corresponding to a metasiiicate. The treatment with the water-glassis preferably carried out in a blending mixer in order to insurethorough mixing. The granular product may be marketed for technicaluses, or passed through a drier heated to a temperature in theneighborhood of 200 C. The fine free-flowing product obtained is of highpurity and corresponds substantially to the formula NazSlOs and as suchis suitable for all industrial uses.

It 60 parts of silica are treated with 123 parts of caustic soda inplace of the proportion in the example previously given, the productformed approaches a sesquisilicate. The product may be marketed as suchor may be subjected to a dressing step corresponding to that previouslydescribed, in which case a sesquisilicate of high purity and solubilityis obtained.

A metasilicate can be formed, if desired, from the technically anhydroussesquisilicate by the addition, preferably in a blending mixer, 01' thenecessary amount of commercial water-glass to bring the ratio of alkalioxide to silica to that corresponding to the metasilicate. The productis preferably treated by passing it rapidly through a drier at atemperature in the neighborhood of 200 C., and a fine, free-flowingproduct is obtained, corresponding substantially to that of technicallyanhydrous metasilicate and which is suitable for all uses.

Ii 60 parts of silica and parts of caustic soda are reacted in the mixerunder the conditions of the first specific example, a productapproaching an orthosllicate in composition is formed. This product isstable, free-flowing, and practically totally soluble, and can be usedcommercially. In order to obtain a technically anhydrous sodiumorthosilicate, the product oi the mixer is heated in a suitable drier asdescribed at temperatures from 300 C. to 600" (3., preferably in theneighborhood of 450 C. The product is also free-flowing and availablefor a l technical uses.

If the silicates produced in accordance with the process of the reactionare to be used as detergents, it may be desirable to incorporate othermaterials possessing somewhat similar properties with the silicates, forexample, trisodium phosphate, disodium phosphate, sodium carbonate, andsodium bicarbonate. The materials may be incorporated with the silicatesat any desired point in the process, depending upon the nature andchemical properties of the mater als included.

While in this specification, there is described a process oi convertinga subsilicate 01' higher ratio of alkali oxide to silicon dioxide to oneof a lower ratio by reaction of the subsilicate with a compound of thetype of water glass, no claims directed to this process are made herein,since such invention is described and claimed in our co-pendingapplication Serial No. 131,748, filed March 18, 1937, as acontinuation-in-part hereof (now Patent No. 2,175,781).

Considerable modification is possible in the proportions of reactivesilica material and caustic alkali employed, as well as in the physicalfactors used in the various steps of the process without departing fromthe essential features of the invention.

We claim:

i.' The process of preparing a solid alkali subsilicate, which comprisesmixing together powdered silica and solid subdivided caustic alkali. insuch molecular proportions that the alkali oxide content of the mixtureis not less than the silicon dioxide content, and in the absence ofwater exceeding 7% of the total weight or the silica and caustic alkali;and heating the mixture to a temperature below the iiusion point of saidcaustic alkali suilicient to cause a vigorous reaction between thesilica and caustic alkali and the liberation of suflicient water to formin a short time a subsilicate, the water content or which does notsubstantially exceed that 01 a monohydrate.

2. ,The process of preparing a solid alkali subsilicate, which comprisesmixing together powdered relatively non-reactive silica and solidsubdivided caustic elkali, in such molecular proportions that the alkalioxide content of the mixture is greater than the silicon dioxidecontent, and in the absence of water exceeding 7% of the total weight ofthe silica and caustic alkali; and heating the mixture to a temperaturebelow the fusion point of said caustic alkali, while stirring,sufficient to cause a vigorous reaction between the silica and causticalkali and the liberation of suflicient water to form in a short time asubsilicate, the water content of which does not substantially exceedthat of a monohydrate.

3. The process of claim 2 wherein the caustic alkali is caustic soda andthe silica is naturallyoccurring, substantially pure silica, and whereinthe temperature is above about C.

4. The process of preparing a solid alkali subsilicate, which comprisesmixing together powdered relatively non-reactive silica and solidsubdivided caustic alkali, in such molecular proportions that the alkalioxide content of the mixture is greater than the silicon dioxidecontent, and in the absence of an appreciable amount of water; andheating the mixture to a temperature below the fusion point of saidcaustic alkali, while stirring, suflicient to cause a vigorous reactionbetween the silica and the caustic alkali and the liberation ofsuflicient water to form in a short time a subsilicate, the watercontent of which does not substantially exceed that of a monohydrate.

5. The process of claim 4 wherein the caustic alkali is caustic soda andthe silica is naturallyoccurring, substantially pure silica, and whereinthe temperature is above about 175 C.

6. The process of preparing a solid alkali subsilicate, which comprisesmixing together powdered relatively non-reactive silica and solidsubdivided caustic alkali, in such molecular proportions that the alkalioxide content oi the mixture is greater than the silicon dioxidecontent, and in the absence of water; and heating the mixture to atemperature below the fusion point of said caustic alkali, whilestirring, sufficient to cause a vigorous reaction between the silica andcaustic alkali to form in a short time a subsilicate.

7. The process of claim 6 wherein the caustic alkali is caustic soda andthe silica is naturallyoccurring, substantially pure silica, and whereinthe temperature is above about 175 C.

8. The process of preparing a solid sodium subsilicate, which comprisesmixing together powdered, naturally-occurring. substantially pure silicaand solid subdivided caustic soda, in such molecular proportions thatthe NazO content of the mixture is greater than the S10: content, and inthe absence of water exceeding 7% of the total weight of the silica andcaustic soda; heating the mixture to a temperature below the fusionpoint oi caustic soda suflicient to cause a vigorous reaction betweenthe silica and caustic soda and the liberation oi sufllcient water toform in a short time a subsllicate, the water content of which does notsubstantially exceed that of a monohydrate; and during said'reactionstirring the mass until a solid granular product is obtained.

9. The process of preparing a solid sodium subsiiicate, which comprisesmixing together powdered naturally-occurring, substantially pure silicaand solid subdivided caustic soda, in such molecular proportions thatthe N820 content of the mixture is greater than the 8102 content, and inthe absence of an appreciable amount of water; heating the mixture to atemperature above about 175 C. and below the fusion point of causticsoda suflicient to cause a vigorous reaction between the caustic sodaand silica and the liberation of suiiicient water to form in a shorttime a subsilicate, the water content of which does not substantiallyexceed that of a monohydrate; and during said reaction stirring the mass.until a solid granular product is obtained.

10. The process of preparing a solid sodium subslllcate, which comprisesmixing together powdered naturally-occurring, substantially pure silicaand solid subdivided caustic soda, in such molecular proportions thatthe HMO content of the mixture is greater than the SiO: content, and inthe absence of water; heating the mixture above about C. and to atemperature below the fusion point of caustic soda sufllcient to cause avigorous reaction between the caustic soda and silica to form in a shorttime a subsilicate; and during said reaction stirring the mass until asolid granular product is obtained.

11. The process of preparing a solid technically anhydrous sodiumsubsilicate, which comprises mixing together powdered silica and solidsubdivided caustic soda, in such molecular proportions that the NaaOcontent of the mixture is greater than the S102 content, and in theabsense of water exceeding 7% of the total weight of the silica andcaustic soda: heating the mixture to a temperature below the fusionpoint of caustic soda, while stirring, sufliclent to cause a vigorousreaction between the caustic soda and silica and the liberation ofsumcient water to form in a short time a subsilicate, the water contentof which does not substantially exceed that of a monohydrate; andthereafter further heating the product at a temperature above 300 andbelow the melting point thereof to cause a further reaction and theconversion of the product into the technically anhydrous truesubsilicate product.

12. The process of preparing a solid technically anhydrous sodiumsub-silicate, which comprises mixing together powderednaturally-occurring, substantially pure silica and solid subdividedcaustlc soda, in such molecular proportions that the NaaO content of themixture is greater than the S102 content, and in the absence of anappreciable amount of water: heating the mixture to a temperature aboveabout 175 C. and below the fusion point of caustic soda, while stirring,sumcient to cause a vigorous reaction between the caustic soda andsilica and the liberation of sumcient water to form in a short time asubsilicate, the water content of which does not substantially exceedthat of a monohydrate; and thereafter further heating the product at atemperature above 800 C. and below the melting point thereof to cause afurther reaction and the conversion of the product into the technicallyanhydrous true subsilicate product.

13. The process of claim 12 wherein the ratio of NaaO to S10: isapproximately 1.5 to 1 and wherein the product formed is substantiallyNasSlzOv.

14. The process of claim 12 wherein the ratio of NaaO to 3102 isapproximately 2 to 1 and wherein the product formed is substantiallyNB4S104.

15. The process of preparing a solid technically anhydrous sodiumsubsilicate, which comprises mixing together powderednaturally-occurring, substantially pure silica and solid subdividedcaustic soda, in such molecular proportions that the N520 content of themixture is greater than the S10: content, and in the absence of waterheating the mixture to a temperature above about 175 C. and below thefusion point of caustic soda, while stirring, suflicient to cause avigorous reaction between the caustic soda and silica to form in a shorttime a subsilicate; and thereafter further heating the product at atemperature above 300 C. and below the melting point thereof to cause afurther reaction and the conversion of the product into the technicallyanhydrous true subsiiicate product.

16. The process of claim 15 wherein the ratio of Nero to SiO: isapproximately 1.5 to 1 and wherein the product formed is substantiallyNaaSlzOv.

17. The process of claim 15 wherein the ratio of N820 to $0: isapproximately 2 to 1 and wherein the product formed is substantiallyNaisiliol- 18. The process of preparing a solid technically anhydroussodium subsilicate, which comprises mixing together powdered,naturally-occurring, substantially pure silica and solid subdividedcaustic soda, in such molecular proportions that the NazO content of themixture is Greater than the S10: content, and in the absence of waterexceeding 7% of the total weight of the silica and caustic soda; heatingthe mixture to a temperature below the fusion point of caustic sodasuflicient to cause a vigorous reaction between the silica and causticsoda and the liberation of sufficient water to form in a short time asubsilicate, the water content of which does not substantially exceedthat of a monohydrate; during said reaction stirring the mass until asolid granular product is obtained; and thereafter further heating theproduct at a temperature above 300 C. and below the melting pointthereof to cause a further reaction and the conversion of the productinto the technically anhydrous true sub-silicate product.

19. The process of claim 2 wherein the temperature is above about 175 C.

CLARENCE W. BURKHART. WALTER S. RIGGB.

CERTIFICATE OF CORRECTION. Reissue No. 21,705. February 1+, 1914.1.

CLARENCE W. BURKHART, ET AL.

It is hereby certified that error appears in the printed specificationof the above numbered patent requiring correction as follows: Page 2,second column, line 59, for "as" read --at; page LL, first column, linek0, claim 10, for the words "above about 175 C. and to a temperature"read -to a temperature above about 175 C. and-; same page, secondcolumn, line l e, claim 17, for "NahSi Oh" read -NahSiOh--; and that thesaid Letters Patent should be read with this correction therein that thesame may conform to the record of the case in the Patent Office.

Signed and sealed this 18th day of March, A. D. 19!;1.

Henry Van Arsdale, (Seal) Acting Commissioner of Patents.

